One known method of detecting an abnormality occurring in fuel cells utilizes an AC impedance of the fuel cells as disclosed in, for example, Japanese Patent Laid-Open No. 2002-367650.
This prior art method measures in advance an AC impedance of a polymer electrolyte fuel cell or a fuel cell stack at a specific frequency in the state of normal power generation and sets the measured AC impedance to a reference impedance value. The method then measures the AC impedance of the fuel cell or the fuel cell stack at the specific frequency during power generation and compares the measured AC impedance with the reference impedance value. When a difference between the measured AC impedance and the reference impedance value exceeds a preset allowable limit, it is determined that some abnormality arises in the fuel cell or the fuel cell stack. A diffusion resistance (concentration polarization) of the fuel cell is specified by an imaginary part of an AC impedance measured at a frequency of 5 Hz. A reaction resistance of the fuel cell is specified by an imaginary part of an AC impedance measured at a frequency of 40 Hz. An ohmic resistance of the fuel cell is specified by a real part of the AC impedance measured at the frequency of 5 Hz.
The AC impedance of the fuel cell is varied not only by performance degradation in the fuel cell but may also be varied even in the normal state with a variation in operating conditions of the fuel cell. In application of fuel cells as an energy supply source of a vehicle, the operating conditions of the fuel cells (for example, the temperature and the power output) significantly vary with a variation in driving state of the vehicle. This leads to a significant variation in AC impedance.
There may be a difficulty in identifying the cause of a large difference between the measured AC impedance and the reference impedance value exceeding the preset allowable limit between the variation of the AC impedance due to the occurrence of some abnormality (performance degradation) in the fuel cell or the fuel cell stack and the variation of the AC impedance due to a variation in operating conditions of the fuel cell.
For example, the reaction resistance in the fuel cell may be increased by degradation of the catalytic activity as well as a temporary decrease of the catalytic activity based on a temperature decrease. The prior art method can not discriminate the performance degradation (abnormality) from the simple temporary performance decrease. The similar difficulties arise for the diffusion resistance and the ohmic resistance.
In the specification hereof, the terminology ‘performance degradation’ means permanent and fatal performance decrease, whereas the terminology ‘temporary performance decrease’ means only temporal performance decrease with a potential for recovery of performance in some conditions.